1. Field of the Invention
The present invention relates to a method for estimating and correcting surface consistent statics in seismic acoustic wave reflection traces and, more particularly, to a method for estimating and correcting surface consistent statics in such traces by using measurements of reflection and refraction signal component arrivals contained in the traces.
2. Discussion of the Prior Art
The determination of source-receiver statics for a group of seismic traces is well known in the art. Source-receiver statics determinations and corrections are particularly widely used when common depth point (CDP) seismic traces are gathered and stacked, e.g., summed, to improve the signal-to-noise ratio of the reflection traces. To obtain the benefits commonly associated with CDP stacking, it is imperative that primary reflection energy components in the various traces of the gather be properly aligned in time before stacking.
Typically, the seismic traces are time shifted, before stacking to account for offsets in the source-receiver pair spacing used to generate the traces. This is commonly referred to as normal moveout (NMO) correction. In addition, elevation statics may be determined and applied to correct the traces to simulate the sources and receivers used to generate the traces being on a flat surface. Thereafter, source-receiving statics are typically estimated and used to further time shift the traces so that they are properly aligned in time for the CDP stacking operation.
FIG. 1 illustrates the differences in source-to-datum travel time S.sub.1 . . . S.sub.n and receiver-to-datum travel time R.sub.1 . . . R.sub.n for a common depth point gather of traces, which are typically corrected for elevation statics, normal moveout (NMO), and source-receiver statics prior to stacking. This is typically done by first correcting for elevation statics and normal moveout and then estimating and correcting for source-receiver statics. To apply the proper amount of source-receiver statics corrections, it is necessary to first determine the source-receiver statics S.sub.i +R.sub.j associated with the source and receiver pair used to generate a trace and then apply this as a time shift to the trace.
The statics S.sub.i and R.sub.j can be estimated by determining or estimating the distance in feet between each of the source S.sub.i and receiver R.sub.j pairs to a datum plane and then dividing this distance by a wave propogation velocity value through the near surface layer l.sub.1, i.e., t=l.sub.1 /v. This would provide some measure of the time offset introduced into a seismic trace by the source and receiver statics. However, it is difficult to precisely determine a velocity value for a particular source or receiver location, since different subsurface formation characteristics have different velocity characteristics and the datum may be located in a region where several formation layers are present between it and the surface, each having its own velocity characteristic. Moreover, if the distance of the source and receiver to the datum is estimated, this introduces additional errors in the source-receiver statics estimations, which are applied as time shift corrections to the individual seismic traces.
Various techniques have been developed for processing the seismic reflection signal traces to determine and correct for source and receiver statics to permit proper CDP trace stacking. Several such techniques are described in the following articles. "Automated Statics Corrections" by Hileman, et al, Geophysical Prospecting, Vol. XVI, pp. 326-358; "Estimation and Correction of Near-Surface Time Anomalies" by Taner, et al, Geophysics, Vol. 39, No. 4 (Aug. 19, 1974), pp. 441-463; "Residual Statics Analysis As A General Linear Inverse Problem", by Wiggins, et al, Geophysics, Vol. 41, No. 5 (October 1976), pp. 922-938.
Many techniques for correcting statics employ the cross-correlation of one trace of a gather, selected as a reference, with the remaining traces to determine the relative time shift between remaining traces and reference trace. The determined time shifts are then applied to the remaining traces to align them in time with the reference trace for stacking. In some techniques, reflection signal time of arrival data is derived and used to determine and apply the statics necessary for proper time alignment of the traces. While several of the known techniques can provide good time alignment results under the right circumstances, it is also possible to obtain poor results and consequent improper trace alignment and subsequent trace stacking.